drm/amd/display: Set irq state only on existing crtcs

Because AMDGPU_CRTC_IRQ_VLINE1 = 6, it expected 6 more crtcs to be
programed with disabled irq state in amdgpu_irq_disable_all. That caused errors and accessed
the wrong memory location.

Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>